\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usetikzlibrary{shapes, calc, decorations}
\usepackage{amsmath,amssymb}

\begin{document}
\begin{preview}
\resizebox{250px}{250px}{
\begin{tikzpicture}[]
    \newcommand\R{1.3cm}

    \draw[fill=gray!2] (0,0) circle (\R);

    % draw the background
    \draw [line width=0.1pt, fill=gray!10]
        (0,0) --
        (170: \R) --
        (45:\R) -- cycle;

    \begin{scope}[color=green]
        \draw[fill=green!15] (0,0) --
                        (170:{\R*0.3}) arc (170:45:{{(\R)*0.3}});
    \end{scope}

    % Kreisbogen
    \begin{scope}[thick, color=red]
        \draw[] (170:\R) arc (170:45:\R);
    \end{scope}

    % Winkel
    \draw[] (0,0) -- (170:\R);
    \draw[] (0,0) -- (45:\R);
    \draw[thick, color=blue] (170:\R) -- (45:\R);

    % draw alpha
    \coordinate[label=$\alpha$] (Alpha) at (-{\R/20}, 0);
    \coordinate[label=$r$] (Alpha) at ({\R/2}, {\R/5});
\end{tikzpicture}
}
\end{preview}
\end{document}
